Urea-SCR (Selective Catalytic Reduction) is a well-known solution for treating the NOx emissions from diesel engines, but requires the exhaust temperatures to be above 200° C. [1]. The heating rate on diesel engines/exhaust can be relatively slow, and this results in a delay between switching the engine on and being able to dose urea and effectively remove NOx by SCR (“cold-start” period). The NOx emissions during this cold start period comprise a large portion of the total emissions during the FTP-75 and NEDC test protocols for example, and this has significant implications for real world driving.
In Europe, the legislated NOx limits for diesel vehicles have been constantly falling since the introduction of the Euro standard, and the current Euro VI limit is 0.080 g/km (September 2014).
One solution is to utilize a “passive NOx adsorbent” (PNA) material upstream of the SCR catalyst, which is capable of storing NOx below 200° C. (i.e., during the cold start period) and then releasing it above this temperature (i.e., once the SCR catalyst is active).
Standard lean NOx trap materials (e.g., Pt/Ba/Al2O3) which require the oxidation of NO to NO2 are useful at higher temperatures but do not tend to store NOx efficiently below 150° C. In this regard, an alternative class of materials are necessary that are more active at lower temperatures (from ambient up to 200° C.).
In addition to the low temperature NOx storage capability, PNA materials must also have suitable thermal stability. Depending on the location (e.g., on DOC), it may experience temperatures up to 800-850° C. (hydrothermal) under high engine load conditions. The PNA will always be upstream of the SCR catalyst but may be downstream of a filter, which could be regenerated actively or passively. Hence the PNA must maintain its low temperature activity after such thermal excursions.
Further to these thermal stability demands, candidate PNA materials should also be robust to the presence of sulfur-containing species in the exhaust gas. This implies that the materials have a relatively low propensity for adsorbing sulfur species, but also tend to de-sulfate under suitable conditions (e.g. preferably below 700° C. in lean conditions).
U.S. Pat. No. 8,105,559 refers to the use of palladium on ceria (Pd—CeO2) as an effective PNA candidate. NOx is allegedly stored effectively at 120° C., 160° C. or 200° C., and is allegedly desorbed almost immediately upon ramping the temperature. However, no data is provided on the effect of sulfur in the feed gas.
U.S. Pat. No. 8,920,756 refers to the use of an Ag/Al2O3 component in combination with another material to create a passive NOx adsorber system. The second material may contain manganese, but only in combination with ceria, and this is likely to be inherently sulfur-intolerant. In addition to this, the function of the second component is to store NOx once the temperature is above 190° C. (NOx during the initial cold start period being stored on the Ag/Al2O3 component).
U.S. Pat. No. 9,687,811 discusses the use of various materials/combinations for use in the PNA application. Specific mention is made of manganese, but this is used/added as a bulk Mn3O4 component (i.e., not part of a solid solution) which is expected to lead to poor thermal stability and low sulfur-tolerance. Further to this, the Mn3O4 component is always added in combination with a ceria component.
Zhao-shun Zhang and co-workers (Appl. Cat. B: Environmental, 165 (2015) 232-244) investigated the addition of manganese into a model lean NOx trap (Pd/Ba/Al2O3). They demonstrated enhanced NO oxidation activity but required temperatures above 300° C. for efficient NOx storage.
Li-Hong Guo and co-workers (Catal. Today, June 2017) also investigated model manganese oxide systems under more relevant NOx storage conditions (i.e., <200° C.) and found that NOx could be stored effectively. However, although MnO2 had the greatest NOx storage capacity, the strong adsorption of NOx meant that desorption was more difficult, and Mn2O3 showed more facile NOx release. So, when designing manganese-containing PNA materials, one should consider the state of the Mn species and the impact of other components of the mixed or composite oxide on this. Oxidation of NO to NO2 is not always beneficial, with surface nitrites being generally less stable than nitrates, and thus more easily desorbed.
U.S. Patent application publication No. 2009/0191108 refers to the use of praseodymia-zirconia mixed oxides (optionally containing ceria) in NOx trapping applications for lean burning internal combustion engines. Although the materials showed improved sulfur-tolerance compared to Ba/Al2O3 reference (after rich regeneration at 550° C.), there is no low temperature activity promoting element (such as a transition metal) and these materials require temperatures of 200-300° C. for suitable NOx storage.
And finally, the palladium-on-zeolite system has received a lot of attention for the PNA application, such as U.S. published patent application No. 2012/0308439. Although efficient low temperature NOx storage is observed, the palladium usage can be quite high (>50 g/ft3) which has cost implications, and these materials also tend to adsorb hydrocarbons which may or may not be advantageous.